NOx emissions from vehicles with internal combustion engines are an environmental problem reognized worldwide. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. In conventional gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel powered vehicles and vehicles with lean-burn gasoline engines, however, the exhaust is too oxygen-rich for three-way catalysts to be effective.
Several solutions have been posed for controlling NOx emissions from diesel powered vehicles and lean-burn gasoline engines. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation, homogenizing fuel-air mixtures, and inducing sparkless ignition can reduce NOx emissions. These techniques alone, however, will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, NOx adsorber-catalysts, and selective catalytic reduction (SCR).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiency with lean-burn catalysts is unacceptably low.
NOx adsorber-catalysts alternately adsorb NOx and catalytically reduce it. The adsorber can be taken offline during regeneration and a reducing atmosphere provided. The adsorbant is generally an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst can be a precious metal, such as Ru. A drawback of this system is that the precious metal catalysts and the adsorbant may be poisoned by sulfur.
SCR involves using ammonia as the reductant. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuousy into the exhaust. SCR can achieve NOx reductions in excess of 90%, however, there is concern over the lack of infrastructure for distributing ammonia or a suitable precursor. SCR also raises concerns relating to the possible release of ammonia into the environment.
U.S. Pat. No. 6,560,958 describes an adsorber-catalyst system in which hydrogen-rich synthesis gas (syn gas), including H2 and CO, is used as a reductant to regenerate the adsorber. The syn gas is produced from diesel fuel in a plasma converter. Periodically, the adsorber is taken offline from the exhaust system and supplied with the syn gas. The adsorber contains a catalyst for SCR of NO2 with syn gas.
U.S. Pat. No. 5,587,137 describes an exhaust system with a zeolite adsorber and a catalyst placed in line. The system is intended for a stoichimetrically fed engine. The adsorber is meant to be active during cold start. After the adsorber and catalyst heat up desorption is induced by making the exhaust stream rich in oxygen. Desorbed products are asserted to undergo conversion over the downstream catalyst in spite of the oxygen-rich conditions. A preferred adsorbant is a Y, Beta, or ZSM-5 zeolite with an SiO2 to Al2O3 mole ratio of about 3 to about 20, ion exchanged with a rare earth metal, chromium, or a combination thereof.
There continues to be a long felt need for reliable, affordable, and effective systems for removing NOx from the exhaust of diesel and lean-burn gasoline engines.